The company, known as NIE Energy until recently, blamed the 18.6% increase on the rising cost of wholesale fuels needed to generate electricity.

The move, which will see the average annual household bill rise by almost £100 to £588, has been described by the Consumer Council as a "massive blow" to customers while the Northern Ireland Independent Retail Trade Association (NIIRTA) has also expressed concern.

Power NI said it was the last main electricity supplier in the UK to increase prices this year.

It stressed the decision had come after tariffs were frozen last year and cut by 15% in 2009. A 60% rise in the cost of wholesale fuels costs has forced the price increase, the supplier said.

Power NI supplies around 720,000 homes and more than 35,000 farm and business customers.

Stephen McCully, Managing Director of Power NI, said: "Our last price increase was three years ago and since then we've managed to either freeze or cut prices, so I'm very disappointed to have to make this announcement. However, we have been faced with steadily rising wholesale gas and oil prices over the last two years and we can't hold our prices any longer."

The tariff rise will come into effect at the start of October.

Consumer Council chief executive Antoinette McKeown said: "This increase will mean that Power NI customers' bills will rise, on average, by £92 per year to £588."

Overall, it will mean that the majority of households in Northern Ireland will have an energy bill of £2,114 per year. This is around £900 a year more than households in Great Britain, most of which rely on gas for heating, she said.